Being black and a woman has not stopped Oprah Winfrey becoming one of the most powerful people in the world and her claim to influence lies on stronger foundations than her ability to get stars such as Tom Cruise over-sharing on her couch.

The importance of an appearance on her talkshow was underlined when the then President-elect Barack Obama was a guest – it was seen as providing a boost to his profile not hers.

She rose to become the world's first black female self-made billionaire from a childhood so poor it sounds like a punchline for a joke – she adopted two cockroaches as pets and wore sackcloth as her grandmother could not afford to buy her clothes.

Her willingness to talk about her years of being sexually abused, her teenage pregnancy and the loss of her baby, her constant battle with her weight and childhood poverty have made her a hero to millions of viewers around the world.

Her endorsements can make careers (books she mentions routinely become bestsellers) and she doesn't always pick perfectly (Jenny McCarthy appeared on her show to explain why she thinks vaccination caused her son's autism), but her support for gay rights, Aids awareness, sexual abuse victims and literacy campaigns are impressive and consistent.

As is her philanthropy: she founded a school in South Africa, Oprah's Angel Network, which gives educational grants, and personally donated $10m to rebuild homes after Hurricane Katrina.

At 57, she's hardly self-effacing – her latest venture is her own television channel to add to a magazine called O, The Oprah Magazine – but few have done as much to put women, poor black ones at that, on the international map.